Die-Sized Atomic Magnetometer and Method of Forming the Magnetometer

ABSTRACT

The cost and size of an atomic magnetometer are reduced by attaching together a first die which integrates together a vapor cell, top and side photo detectors, and processing electronics, a second die which integrates together an optics package and a heater for the vapor cell, and a third die which integrates together a VCSEL, a heater for the VCSEL, and control electronics.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to atomic magnetometers and, moreparticularly, to a die-sized atomic magnetometer and a method of formingthe magnetometer.

2. Description of the Related Art

An atomic magnetometer is a device that measures the strength of amagnetic field by determining a frequency known as the Larmor frequency.The Larmor frequency, in turn, is the frequency of the magnetic momentof a contained group of in-phase spinning alkali atoms moving inprecession in response to the magnetic field. A magnetic field strengthB is defined by the equation B=hv_(L)/γ, where h is Plank's constant,hv_(L) is the Larmor frequency, and γ is the gyromagnetic ratio (e.g., 7Hz/nT for ⁸⁷Rb and 3.5 Hz/nT for Cs).

FIG. 1 shows a block diagram that illustrates an example of a prior artatomic magnetometer 100. As shown in FIG. 1, atomic magnetometer 100includes a vertical cavity surface emitting laser (VCSEL) 110, and anoptics package 112 that lies above VCSEL 110. Further, atomicmagnetometer 100 also includes a vapor cell 114 that lies above opticspackage 112, and a photo detector 116 that lies above vapor cell 114.

In addition, vapor cell 114 contains a gas 118 that includes alkaliatoms, which have a single electron in the outer shell, and bufferatoms, which reduce collisions between the alkali atoms and the innersurface of vapor cell 114. For example, vapor cell gas is commonlyimplemented with alkali atoms such as ⁸⁵Rb atoms, ⁸⁷Rb atoms, and Csatoms, and buffer atoms such as N₂. Further, atomic magnetometer 100 canoptionally include a lower coil 120 and an upper coil 122 that lie belowand above vapor cell 114.

In operation, VCSEL 110 outputs light which is attenuated and circularlypolarized by optics package 112. The circularly polarized light outputby optics package 112 is then directed into vapor cell 114. The lightoutput by VCSEL 110 is tuned to a frequency which, when circularlypolarized, is absorbed by the single electrons in the outer shells ofthe alkali atoms in the gas 118 contained within vapor cell 114. Forexample, VCSEL 110 can be tuned to output light with a wavelength of 795nm which, after being circularly polarized, is absorbed by the singleelectrons in the outer shells of ⁸⁷Rb atoms.

When the single electron in the outer shell of an alkali atom absorbslight energy, the electron transitions to a higher energy level, andthen falls back to one of a number of energy levels (Zeeman sublevelswithin the hyperfine energy levels) that are associated with the outershell. When falling back, the electron emits a photon in a randomdirection, and always falls back to the highest energy level that isassociated with the outer shell.

When the electron falls back to the highest energy level that isassociated with the outer shell, the electron is no longer capable ofabsorbing any light energy. To again reabsorb light, additional energyat the Larmor frequency must be applied to the electron. The additionalenergy at the Larmor frequency causes the electron to drop to a lowerenergy level that is associated with the outer shell where the electroncan again absorb light energy.

Thus, the photons that pass out of vapor cell 114 into photo detector116 include a non-absorption component, which represents the lightoutput by VCSEL 110 that was not absorbed by the electrons in the outershell of the gas 118 within vapor cell 114, and an emission component,which represents the photons that are randomly emitted by the fallingelectrons. Photo detector 116 detects these photons, and generates anoutput signal that has both a non-absorption component and an emissioncomponent.

Two of the common approaches to adding additional energy at the Larmorfrequency are the Bell-Bloom (BB) technique and the M_(X) technique. Inthe BB technique, the light output by VCSEL 110 is modulated by afrequency that is swept across a range of frequencies. When the lightoutput by VCSEL 110 is frequency modulated at the Larmor frequency, theelectrons drop to a lower energy level and begin reabsorbing lightenergy, which causes a noticeable dip in the intensity of light receivedby photo detector 116. Thus, the Larmor frequency can be determined bydetermining the modulated frequency that caused the intensity of thereceived light to dip.

In the M_(X) technique, an RF signal is applied to the lower and uppercoils 120 and 122 to create an alternating magnetic field that isaligned with the longitudinal axis of the light emitted by VCSEL 110,while the frequency of the RF signal is swept across a range offrequencies. When the frequency of the RF signal becomes equal to theLarmor frequency, the electrons drop to a lower energy level and beginreabsorbing light energy, which causes a noticeable dip in the intensityof light received by photo detector 116. Thus, the Larmor frequency canbe determined by determining the RF frequency that caused the intensityof the received light to dip.

Two of the drawbacks of conventional atomic magnetometers are size andcost, which then limit the types of applications where atomicmagnetometers can be commercially utilized. Thus, there is a need fordie-sized atomic magnetometers which can be mass produced inconventional integrated circuit fabrication facilities, thereby reducingboth size and cost and significantly increasing the types ofapplications where atomic magnetometers can be commercially utilized.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating an example of a prior art atomicmagnetometer 100.

FIG. 2A is a cross-sectional view illustrating an example of a die-sizedatomic magnetometer 200 in accordance with the present invention.

FIG. 2B is a schematic diagram illustrating an example of electroniccircuit 230E in accordance with the present invention.

FIG. 3A is a cross-sectional view illustrating an example of a die-sizedatomic magnetometer 300 in accordance with an alternate embodiment ofthe present invention.

FIG. 3B is a plan view illustrating an example of the RF coils inaccordance with the present invention.

FIG. 3C is a flow chart illustrating an example of a method of operatingatomic magnetometer 300 in accordance with the present invention.

FIG. 4 is a cross-sectional view illustrating an example of a die-sizedatomic magnetometer 400 in accordance with an alternate embodiment ofthe present invention.

FIGS. 5A-5U are a series of cross-sectional views illustrating anexample of a method of forming a first wafer in accordance with thepresent invention.

FIGS. 6A-6I are a series of cross-sectional views illustrating anexample of a method of forming second wafer in accordance with thepresent invention.

FIG. 7 is a cross-sectional view illustrating an example of theformation of a stacked wafer 700 in accordance with the presentinvention.

FIG. 8 is a cross-sectional view illustrating an example of theformation of a stacked die 800 in accordance with the present invention.

FIGS. 9A-9Q are a series of cross-sectional views illustrating anexample of a method of forming a third wafer in accordance with thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 2A shows a cross-sectional view that illustrates an example of adie-sized atomic magnetometer 200 in accordance with the presentinvention. As shown in FIG. 2A, atomic magnetometer 200 includes a firstdie 210 that has a top surface 210T, a bottom surface 2106, and a firstdie cavity 212 that extends into first die 210 from the bottom surface210B of first die 210. First die cavity 212, in turn, has a top surface212T and a side wall surface 212S. The top surface 212T of first diecavity 212 lies below and spaced apart from the top surface 210T offirst die 210.

As further shown in FIG. 2A, first die 210 includes a firstsemiconductor structure 214 that has a top surface 214T, a bottomsurface 214B, and a first semiconductor cavity 216 that extends intofirst semiconductor structure 214 from the bottom surface 214B of firstsemiconductor structure 214. First semiconductor cavity 216, in turn,has a top surface 216T and a side wall surface 216S. The top surface216T of first semiconductor cavity 216 lies below and spaced apart fromthe top surface 214T of first semiconductor structure 214.

First die 210 also includes a heat spreader 218 that touches the bottomsurface 214B of first semiconductor structure 214 and the top surface216T and the side wall surface 216S of first semiconductor cavity 216 toline first semiconductor cavity 216. Heat spreader 218 is highlythermally conductive, and has a thermal conductivity that issubstantially greater than copper, which has a thermal conductivity thatis substantially greater than single-crystal silicon.

In the present example, heat spreader 118 is implemented with an oxidelayer 220A and an overlying diamond layer 220B. Diamond, in turn, has athermal conductivity that is 5-10× greater than copper. Further, thebottom surface of heat spreader 218 (or the exterior surface of diamondlayer 220B) forms the bottom surface 210B of first die 210. Heatspreader 118 can also optionally include other or additional layers to,for example, improve adhesion, provide greater thermal conductivity, orto reduce the effect of atomic collisions with the exterior surface ofheat spreader 118 within the cavity.

In the FIG. 2A example, first semiconductor structure 214 is implementedwith a silicon-on-insulator (SOI) structure that has a p-typesingle-crystal silicon bulk region 214L, an insulator 214I that touchesthe top surface of bulk region 214L, and a p-type single-crystal siliconactive region 214A that touches the top surface of insulator 2141. Firstsemiconductor cavity 216 extends through bulk region 214L and insulator214I to expose the bottom surface of active region 214A. As a result,the top surface 216T of first semiconductor cavity 216 is the exposedbottom surface of active region 214A. Alternately, first semiconductorcavity 214 can only extend through bulk region 214L. In this case, thetop surface 216T of first semiconductor cavity 216 is the exposed bottomsurface of insulator 214I.

First semiconductor structure 214 also has an upper photo detector 222that lies vertically between the top surface 214T of first semiconductorstructure 214 and the top surface 216T of first semiconductor cavity216. Upper photo detector 222 has a p-type well 222W and an n-typeregion 222R that touches first well 222W.

The dopant concentration of p-type well 222W is greater than the dopantconcentration of p-type active region 214A. Further, in the FIG. 2Aexample, upper photo detector 222 also has a thin p+ region 222H thattouches n-type region 222R. Thin p+ region 222H reduces surfacerecombination.

In addition, first semiconductor structure 214 has a side wall photodetector 224 that lies horizontally adjacent to and touches the sidewall surface 216S of first semiconductor cavity 216. Side wall photodetector 224 has a p-type well 224W and an n-type region 224R thattouches n-type well 224W.

The dopant concentration of p-type well 224W is greater than the dopantconcentration of p-type bulk region 214L. Further, in the FIG. 2Aexample, side wall photo detector 224 also has a thin p+ region 224Hthat touches n-type region 224R. Thin p+ region 224H reduces surfacerecombination.

First semiconductor structure 214 additionally has a number of firstphoto detector contact structures 226-1 that each extends from the topsurface 214T of first semiconductor structure 214 into firstsemiconductor structure 214 to make electrical connections with p-typewell 222W, and a number of second photo detector contact structures226-2 that each extends from the top surface 214T of first semiconductorstructure 214 into first semiconductor structure 214 to make electricalconnections with n-type region 222R.

First semiconductor structure 214 further has a number of third photodetector contact structures 226-3 that each extends from the top surface214T of first semiconductor structure 214 into first semiconductorstructure 214 to make electrical connections with p-type well 224W, anda number of fourth photo detector contact structures 226-4 that eachextends from the top surface 214T of first semiconductor structure 214into first semiconductor structure 214 to make electrical connectionswith n-type region 224R. (Contact structure 226-2 and contact structure226-3 are shown with dashed lines to indicate that the structures lieabove and below the plane of the page.) In the present example, each ofthe photo detector contact structures 226 is implemented with an innerlayer of oxide and an outer layer of metal which, in turn, can includetungsten.

In addition, first semiconductor structure 214 has a p+ contact region228-1 that touches p-well 222W and contact structure 226-1, and an n+contact region 228-2 that touches n-type region 222R and contactstructure 226-2. First semiconductor structure 214 also has a p+ contactregion 228-3 that touches p-well 224W and contact structure 226-3, andan n+ contact region 228-4 that touches n-type region 224R and contactstructure 226-4.

Further, first semiconductor structure 214 has a number of circuitelements 230 that are formed in and on the top surface 214T of firstsemiconductor structure 214. The circuit elements 230 includetransistors, resistors, capacitors, diodes and similar circuit devices.(For simplicity, only one NMOS transistor 230, which has an n-typesource 230S and an n-type drain 230D spaced apart by a p-type channel230C, a gate oxide layer 230G that touches the top surface 214T of firstsemiconductor structure 214 over channel 230C, and a gate 230R thattouches oxide layer 230G over channel 230C, is illustrated.) Whenelectrically connected together, the circuit elements 230 form anelectronic circuit 230E that processes the signals output by upper photodetector 222 and side wall photo detector 224.

FIG. 2B shows a schematic diagram that illustrates an example ofelectronic circuit 230E in accordance with the present invention. Asshown in FIG. 2B, photo detector 222 and side wall photo detector 224generate photo detect signals D1 and D2, respectively, which representdetected light, while electronic circuit 230E includes amplifiers A1 andA2 that amplify the photo detect signals D1 and D2, respectively, togenerate the amplified photo detect signals AD1 and AD2.

In addition, electronic circuit 230E includes a differential paircircuit 230P that has a first transistor Q1 with a base connected toreceive the amplified photo detect signal AD1, and a second transistorQ2 with a base connected to receive the amplified photo detect signalAD2. Further, first and second transistors Q1 and Q2 each have acollector that is connected to a power supply voltage VDD by way of aresistive element R, and emitters that are connected together, and to acurrent source CS. The output is taken at the collector of transistorQ2.

Referring again to FIG. 2A, first die 210 also has an interconnectstructure 232 that touches the top surface 214T of first semiconductorstructure 214 and the circuit elements 230. Interconnect structure 232includes a non-conductive region 232N, and contacts 232C that extendthrough region 232N to make electrical connections with the circuitelements 230 and the first, second, third, and fourth photo detectorcontact structures 226-1, 226-2, 226-3, and 226-4. In addition,interconnect structure 232 includes a number of metal-1 traces 232M thatlie on region 232N and touch the contacts 232C, and a passivation layer232P that covers non-conductive region 232N and the metal-1 traces 232M.

Interconnect structure 232 electrically connects the circuit elements230 to the first, second, third, and fourth photo detector contactstructures 226-1, 226-2, 226-3, and 226-4. In addition, portions of themetal-1 traces 232M are exposed to form metal bond pads 232B forexternal electrical connections. (Although interconnect structure 232 isshown with a single metal layer, additional metal layers can also beused.) Further, first die 210 can optionally include a mirror 234 thattouches the top surface of interconnect structure 232 and lies overfirst die cavity 212 and first semiconductor cavity 216.

As further shown in FIG. 2A, atomic magnetometer 200 also includes asecond die 240 that has a top surface 240T, a bottom surface 240B, and asecond die cavity 242 that extends into second die 240 from the bottomsurface 240B of second die 240. Second die cavity 242, in turn, has atop surface 242T and a side wall surface 242S.

The top surface 242T of second die cavity 242 lies below and spacedapart from the top surface 240T of second die 240. In addition, the topsurface 240T of second die 240 is attached to the bottom surface 210B offirst die 210. The top surface 240T of second die 240 touches the bottomsurface 210B of first die 210 to close first die cavity 212 and form avapor cell 244, which is hermetically sealed.

Atomic magnetometer 200 further includes a gas 246 that is containedwithin hermetically-sealed vapor cell 244. Gas 246, in turn, includesalkali atoms and buffer atoms. For example, gas 246 can be implementedwith alkali atoms such as ⁸⁵Rb atoms, ⁸⁷Rb atoms, or Cs atoms, andbuffer atoms such as N₂.

As additionally shown in FIG. 2A, second die 240 includes a secondsemiconductor structure 250 that has a top surface 250T, a bottomsurface 250B, and a second semiconductor cavity 252 that extends intosecond semiconductor structure 250 from the bottom surface 250B ofsecond semiconductor structure 250.

Second semiconductor cavity 252, in turn, has a top surface 252T thatcoincides with the top surface 242T of second die cavity 242, and a sidewall surface 252S that coincides with the side wall surface 242S ofsecond die cavity 242. In addition, the bottom surface 250B of secondsemiconductor structure 250 coincides with the bottom surface 240B ofsecond die 240. In the FIG. 2A example, second semiconductor structure250 is implemented with a non-conductive material, such as borosilicateglass (BSG).

Second semiconductor structure 250 also has a heating element 254 and apair of bond pads 256 that are electrically connected to the ends ofheating element 254. (Only one portion of heating element 254 and onebond pad 256 are shown for simplicity.) In the FIG. 2A example, heatingelement 254 is implemented with a strip of undoped polysilicon that islaid out to minimize the magnetic field that is generated by currentflowing through heating element 254. For example, heating element 254can be laid out in long parallel strips with alternate ends connectedtogether to form a serpentine pattern. In addition, the bond pads 256are implemented with an undoped polysilicon outer layer 256L and ametallic inner region 256M.

As shown, the top surfaces of heating element 254 and the bond pads 256lie in the same plane P as the top surface 250T of second semiconductorstructure 250, while the bottom surfaces of heating element 254 and thebond pads 256 are vertically spaced apart from the bottom surface 250Bof second semiconductor structure 250.

As further shown in FIG. 2A, second die 240 also includes an opticspackage 260 that is attached to the top surface 242T of second diecavity 242 (which is the top surface 252T of second semiconductor cavity252). Optics package 260 can be attached with drops 260G of aconventional glue or die attach adhesive placed at the corners of opticspackage 260.

Optics package 260 outputs circularly polarized light in response tolight received from a light source, and can be implemented with anyarrangement that outputs circularly polarized light. In the FIG. 2Aexample, optics package 260 includes an attenuator 260A that reduces theintensity of the input light, a linear polarizer 260L that linearlypolarizes the light output from attenuator 260A, and a quarter waveplate circular polarizer 260C that circularly polarizes the light outputfrom linear polarizer 260L. Optics package 260 is commercially availablefrom a number of sources, such as Thorlabs (www.thorlabs.com) or CVIMelles Griot (www.cvimellesgriot), which provide optics packages to meetcustomer specified requirements for the layers and exterior dimensions.(Thorlabs NE220B is an attenuator, Thorlabs LPVIS100 is a linearpolarizer, and CVI Melles Griot QWPO-895-15-4 is a circular polarizer.)

In addition, second die 240 also includes a heat spreader 262 thattouches the top surface 250T of second semiconductor structure 250. Heatspreader 262 is highly thermally conductive, and has a thermalconductivity that is substantially greater than copper, which has athermal conductivity that is substantially greater than BSG.

In the present example, heat spreader 262 is implemented with a diamondlayer. Diamond, in turn, has a thermal conductivity that is 5-10×greater than copper. Further, the top surface of heat spreader 262 (orthe exterior surface of the diamond layer) forms the top surface 240T ofsecond die 240. Heat spreader 262 can also optionally include other oradditional layers to, for example, improve adhesion, provide greaterthermal conductivity, or to reduce the effect of atomic collisions withthe exterior surface of heat spreader 262 within the cavity. Thus, whenthe outer surfaces of heat spreader 118 and heat spreader 262 are bothdiamond, all of the interior surfaces of hermetically-sealed vapor cell244 are implemented with the same material. Alternately, heat spreader262 can be omitted.

As also shown in FIG. 2A, atomic magnetometer 200 further includes athird die 270 that has a top surface 270T, a bottom surface 270B, and athird die cavity 272 that extends into third die 270 from the topsurface 270T of third die 270 to expose a laser light source. Inaddition, the top surface 270T of third die 270 is attached to thebottom surface 240B of second die 240 with drops 273 of a conventionalglue or die attach adhesive to vertically align the first, second, andthird die cavities 212, 242, and 272.

As further shown in FIG. 2A, third die 270 includes a thirdsemiconductor structure 274 that has a top surface 274T, and a bottomsurface 274B that coincides with the bottom surface 270B of third die270. Third semiconductor structure 274 also has a third semiconductorcavity 276 and a fourth semiconductor cavity 278 that each extends intothird semiconductor structure 274 from the top surface 274T of thirdsemiconductor structure 274.

Third semiconductor cavity 276 has a bottom surface 276B and a side wallsurface 276S, while fourth semiconductor cavity 278 has a bottom surface278B and a side wall surface 278S. In the FIG. 2A example, thirdsemiconductor structure 274 is implemented with a conventional substratematerial, such as single crystal silicon.

Third semiconductor structure 274 also has a heating element 280. In theFIG. 2A example, heating element 280 is implemented with an insulatingoxide outer layer 2801, and a strip of undoped polysilicon 280L that islaid out to minimize the magnetic field that is generated by currentflowing through heating element 280. For example, heating element 280can be laid out in long parallel strips with alternate ends connectedtogether to form a serpentine pattern. As shown, the top surface ofheating element 280 lies in the same plane L as the top surface 274T ofthird semiconductor structure 274, while the bottom surface of heatingelement 280 is vertically spaced apart from the bottom surface 274B ofthird semiconductor structure 274.

Third semiconductor structure 274 also includes a temperature sensor 282that has a top surface which lies in the same plane L as the top surface274T of third semiconductor structure 274. In the FIG. 2A example,temperature sensor 282 is implemented with a diode, which has a p-typewell 282W and an n-type region 282R. The current through a diode variesin response to the temperature of the diode. (Although only onetemperature sensor 282 is illustrated, additional temperature sensorscan also be used.)

As further shown in FIG. 2A, third die 270 also includes a heat spreader284 that touches the top surface 274T of third semiconductor structure274, the bottom surface 276B and the side wall surface 276S of thirdsemiconductor cavity 276 to line third semiconductor cavity 276, and thebottom surface 278B and the side wall surface 278S of fourthsemiconductor cavity 278 to line fourth semiconductor cavity 278. Heatspreader 284 is highly thermally conductive, and has a thermalconductivity that is substantially greater than copper, which has athermal conductivity that is substantially greater than single crystalsilicon.

In the present example, heat spreader 284 is implemented with a diamondlayer. Diamond, in turn, has a thermal conductivity that is 5-10×greater than copper. Heat spreader 284 can also optionally include otheror additional layers to, for example, improve adhesion or providegreater thermal conductivity.

In the present example, the heat spreaders 218, 262, and 284 are alsoelectrical isolators. As a result, heat spreader 284 has a number ofopenings that expose portions of heating element 280, and the contactregions of p-type well 282W and n-type region 282R. The openings,however, provide substantially no interruption to the thermal flow fromheating element 280 to the portion of heat spreader 284 that lines thirdsemiconductor cavity 276. In addition, the region of heat spreader 284that lines fourth semiconductor cavity 278 can be optionally removed andreplaced by a less thermally conductive material.

In addition, third die 270 has a conventionally-fabricated verticalcavity surface emitting laser (VCSEL) 286 that is attached to the bottomsurface 276B of third semiconductor cavity 276 by an adhesive layer 288.Adhesive layer 288 can be implemented with a conventional glue or dieattach adhesive. VCSEL 286 is commercially available from a number ofsources, such as Princeton Optronics (www.princetonoptronics.com) orM-Com (www.m-com.com.tw/en), which provide VCSELs to meet customerspecified requirements for light frequency, tuning range, power rating,and exterior dimensions.

Third die 270 also has an integrated circuit 290 that is attached to thetop surface 278T of fourth semiconductor cavity 278 by an adhesive layer292. Adhesive layer 292 can be implemented with a conventional glue ordie attach adhesive. Integrated circuit 290 is aconventionally-fabricated die that includes one or more conventionalelectronic circuits that control the current flow through heatingelements 254 and 280, detect the current flowing through temperaturesensor 282, control the operation of VCSEL 286, and process the signalsoutput by electronic circuit 230E.

Further, third die 270 has an interconnect structure 294 that touchesthe top surface of heat spreader 284. Interconnect structure 294, whichincludes metal-1 traces 294M and a non-conductive region 294P,electrically connects integrated circuit 290 to heating element 280,temperature sensor 282, and VCSEL 286.

As shown in FIG. 2A, interconnect structure 294 has an opening 294Z thatextends through non-conductive region 294P to expose the laser output ofVCSEL 286, and glue openings 294G. In addition, portions of the metal-1traces 294M are exposed to form metal bond pads 294B for externalelectrical connections. (Although interconnect structure 294 is shownwith a single metal layer, additional metal layers can also be used.)

As further shown in FIG. 2A, atomic magnetometer 200 additionallyincludes a number of bonding wires 296 which can electrically connectthe metal bond pads (e.g., 232B, 256, and 294B) on the first, second,and third dice 210, 240, and 270 together as needed, and to externalconnections. (Only one bonding wire 296 is shown in the FIG. 2A examplefor simplicity.)

In the operation of atomic magnetometer 200, VCSEL 286 outputs a lightbeam B with a wavelength which, after being circularly polarized byoptics package 260, is absorbed by the single electrons in the outershells of the alkali atoms of the gas 246 within hermetically sealedvapor cell 244. The electrons jump to a higher energy level as theelectrons absorb light energy, and then fall back emitting photons inrandom directions. In the FIG. 2A example, the Bell-Bloom (BB) techniqueis used to frequency modulate the light to identify the Larmor frequencyand enable the electrons to again re-absorb light energy.

In the present invention, the light output by vapor cell 244 is detectednot only by upper photo detector 222, but also by side wall photodetector 224. The photons that pass out of vapor cell 244 into upperphoto detector 222 include a non-absorption component, which representsthe light output by VCSEL 286 that was not absorbed by the electrons inthe outer shell of the gas 246 within vapor cell 244, and an emissioncomponent, which represents the photons which are randomly emitted bythe falling electrons.

Thus, upper photo detector 222 measures non-absorbed photons andemission photons to determine a total number of measured photons. Thephoto detect signal D1 shown in FIG. 2B generated by upper photodetector 222 represents the total number of measured photons and,thereby, has a non-absorption component and an emission component. As aresult, the amplified photo detect signal AD1 shown in FIG. 2B also hasboth a non-absorption component and an emission component.

However, the photons that pass out of vapor cell 244 into side wallphoto detector 224 are substantially only the photons that are randomlyemitted by the falling electrons. Thus, side wall photo detector 224measures substantially only emission photons to determine a total numberof measured photons. The photo detect signal D2 shown in FIG. 2Bgenerated by side wall photo detector 224 represents the total number ofmeasured photons and, thereby, has substantially only an emissioncomponent. As a result, the amplified photo detect signal AD2 shown inFIG. 2B also has substantially only an emission component.

Differential pair circuit 230P shown in FIG. 2B subtracts the amplifiedphoto detect signal AD2, which has substantially only an emissioncomponent, from the amplified photo detect signal AD1, which has both anon-absorption component and an emission component, to generate anoutput signal DS.

The emission photons are emitted randomly. As a result, the emissioncomponent of the amplified photo detect signal AD1 and the emissioncomponent of the amplified photo detect signal AD2 are, on average,equal over time. As a result, the output signal DS representssubstantially only the non-absorption component.

As the electrons drop to a lower energy level and begin reabsorbinglight energy, the re-absorption causes a noticeable dip in the intensityof the output signal DS. The Larmor frequency can then be determined bydetermining the modulated frequency that caused the intensity of theoutput signal DS to dip.

Thus, one of the advantages of the present invention is that side wallphoto detector 224 provides an additional source of photonic informationwhich, in turn, allows the present invention to provide an output signalDS that is more accurate than a prior art output signal that includesboth a non-absorption component and an emission component. Even if theemission component of the amplified photo detect signal AD1 and theemission component of the amplified photo detect signal AD2 are notequal at a given moment, the output signal DS is still more accuratethan the prior art output signal.

In addition, the increased accuracy of the output signal DS alsoincreases the signal-to-noise ratio. Further, any systemic variationsequally effect both the photo detect signals D1 and D2, and thus aresubtracted out of the output signal DS by differential pair circuit230P. Another advantage of the present invention is that having thephoto detectors 222 and 224 and the circuit elements 230 integrated intothe same substrate material minimizes parasitic signal attenuation fromthe photo detectors 222 and 224 to the amplifiers A1 and A2 withinelectronic circuit 230E. In addition, the use of an SOI wafer improvesphoto carrier signal-to-noise performance.

A further advantage of the present invention is that the efficient heattransfer provided by the diamond layers of the heat spreaders 218, 262,and 284 allows less current to be used by the heating elements 254 and280 as well as allowing the heating elements 254 and 280 to be remotelylocated from vapor cell 244 and VCSEL 286, both of which require heatfor conventional operation.

Less current and the remote location of the heating elements 254 and 280allows the magnetic interference generated by current flowing throughthe heating elements 254 and 280 to be reduced. In addition, alkaliatoms have less of a reaction with diamond than oxide or silicon whenthe alkali atoms bump the interior sides of vapor cell 244.

FIG. 3A shows a cross-sectional view that illustrates an example of adie-sized atomic magnetometer 300 in accordance with an alternateembodiment of the present invention. Atomic magnetometer 300 is similarto atomic magnetometer 200 and, as a result, utilizes the same referencenumerals to designate the elements that are common to bothmagnetometers.

As shown in FIG. 3A, atomic magnetometer 300 differs from atomicmagnetometer 200 in that atomic magnetometer 300 includes a lower RFcoil 310 which, in the present example, has a single loop. Lower RF coil310 touches second semiconductor structure 250, and has a top surfacethat lies in the plane P. In the FIG. 3A example, lower RF coil 310 canbe implemented with an undoped polysilicon outer layer 310L and ametallic inner region 310M, along with a pair of exposed ends thatfunction as bond pads.

Atomic magnetometer 300 also differs from atomic magnetometer 200 inthat atomic magnetometer 300 includes a number of upper RF coils 312which each, in the present example, has a single loop. In the FIG. 3Aexample, four upper RF coils 312-1, 312-2, 312-3, and 312-4 areutilized. The upper RF coils 312 touch the top surface of interconnectstructure 232. (Upper RF coil 312-2 and upper RF coil 312-3 are shownwith dashed lines to indicate that the structures lie above and belowthe plane of the page.) Atomic magnetometer 300 also includes anon-conductive layer 314 that lies over the upper RF coils 312, exceptfor a pair of ends on each coil which function as bond pads.

FIG. 3B shows a plan view that illustrates an example of the RF coils inaccordance with the present invention. As shown in FIG. 3B, lower RFcoil 310 substantially surrounds the longitudinal axis of a beam oflaser light B that is output by VCSEL 286, while none of the upper RFcoils 312-1, 312-2, 312-3, and 312-4 surround any portion of thelongitudinal axis of laser light beam B. Each of the loops of the upperRF coils 312-1, 312-2, 312-3, and 312-4 lies in a plane that lies abovethe plane that includes lower RF coil 310.

FIG. 3C shows a flow chart that illustrates an example of a method ofoperating atomic magnetometer 300 in accordance with the presentinvention. As shown in FIG. 3B, the method begins in 320 by generatingan RF current flow through lower RF coil 310, and each of the upper RFcoils 312-1, 312-2, 312-3, and 312-4.

After this, the method moves to 322 where the amplitude of the RFcurrent flowing through the upper RF coils 312-1, 312-2, 312-3, and312-4 is varied to steer a magnetic field axis of a magnetic field thatresults from the RF current flow through lower RF coil 310 and the upperRF coils 312-1, 312-2, 312-3, and 312-4. The magnetic field axis issteered to be parallel with the longitudinal axis of a laser light beamB generated by VCSEL 286 (which passes through optics package 260 andvapor cell 244).

As a result, the current through the four upper RF coils 312-1, 312-2,312-3, and 312-4 can be individually varied to steer the axis of theoverall magnetic field that results from current flowing through lowerRF coil 310 and the currents flowing through the four upper RF coils312-1, 312-2, 312-3, and 312-4.

Thus, one of the advantages of the present invention is that atomicmagnetometer 300 allows the axis of the overall magnetic field to bealigned with the axis of the laser light output by VCSEL 286. Atomicmagnetometer 300 otherwise operates in the same manner as atomicmagnetometer 200, except that the M_(X) technique is used to identifythe Larmor frequency rather than the BB technique utilized by atomicmagnetometer 200, i.e., the RF frequency applied to the lower and upperRF coils 310 and 312 is swept across a range of frequencies. Anotheradvantage of atomic magnetometer 300 is that atomic magnetometer 300 canbe implemented to operate using either the M_(X) technique or the BBtechnique.

FIG. 4 shows a cross-sectional view that illustrates an example of adie-sized atomic magnetometer 400 in accordance with an alternateembodiment of the present invention. Atomic magnetometer 400 is similarto atomic magnetometer 200 and, as a result, utilizes the same referencenumerals to designate the elements that are common to bothmagnetometers.

As shown in FIG. 4, atomic magnetometer 400 is identical to atomicmagnetometer 200, except that atomic magnetometer 400 also includes a anupper ferrite block 410 that is attached to the top surface ofinterconnect structure 232 by an adhesive layer 412. Adhesive layer 412can be implemented with a conventional glue or die attach adhesive.

As further shown in FIG. 4, atomic magnetometer 400 additionallyincludes a lower ferrite block 413, while the third semiconductorstructure 274 of atomic magnetometer 400 includes a fifth semiconductorcavity 414 that extends into third semiconductor structure 274 from thetop surface 274T of third semiconductor structure 274. Fifthsemiconductor cavity 414 has a bottom surface 414B and a side wallsurface 414S. Lower ferrite block 413, in turn, is attached to thebottom surface 414B of fifth semiconductor cavity 414 by an adhesivelayer 416. Adhesive layer 416 can be implemented with a conventionalglue or die attach adhesive.

Upper ferrite block 410 and lower ferrite block 413 are positioned toestablish a magnetic field that lies approximately 45° off of the axisof the laser beam output from VCSEL 286, which provides maximumsensitivity. Thus, one of the advantages of atomic magnetometer 400 isthat atomic magnetometer 400 provides the maximum sensitivity.

FIGS. 5A-5U show a series of cross-sectional views that illustrate anexample of a method of forming a first wafer in accordance with thepresent invention. As shown in FIG. 5A, the method utilizes aconventionally formed silicon-on-insulator (SOI) wafer 500 that includesa p-type single-crystal silicon bulk 510, an insulator 512 that touchesp-type bulk 510, and a p-type single-crystal silicon active region 514that touches insulator 512. In the present example, SOI wafer 500 isapproximately 1 mm thick and polished on both sides, while active region514 has a thickness in the range of 10-100 μm. The method includes bothfront-side and back-side processing which can be performed in eitherorder.

As further shown in FIG. 5A, the method begins with the back-sideprocessing by inverting SOI wafer 500, thinning the p-type bulk 510 asneeded, and then forming a mask 516 on p-type bulk 510 in a conventionalmanner. After mask 516 has been formed, the exposed regions of p-typebulk 510 are etched until the surface of insulator 512 is exposed. Inthe present example, the exposed regions of insulator 512 are thenetched to expose p-type active region 514 and form a semiconductorcavity 520. Mask 516 is then removed in a conventional manner.

Following this, as shown in FIG. 5B, a layer of oxide is grown on theexposed regions of p-type bulk 510. After this, the layer of oxide isremoved using a conventional dip, and then re-grown to form a thin oxidelayer 522 approximately 500 Å thick. Oxide layer 522 lines cavity 520 toform a lined cavity 524.

Next, as shown in FIG. 5C, a p-type dopant, such as boron, is implantedwithout masking through oxide layer 522 and driven in to form a p− well526. P− well 526 has a dopant concentration that is greater than thedopant concentration of p-type active region 514. Once p− well 526 hasbeen formed, a p-type dopant is then angle implanted in a conventionalmanner without masking and driven in to form a p− side-wall well 528.P-side-wall well 528 has a dopant concentration that is greater than thedopant concentration of p-type bulk 510.

After p− side-wall well 528 has been formed, an n-type material, such asphosphorous, is implanted with a lower implant energy without maskingthrough oxide layer 522 and driven in to form an n-type region 530 in p−well 526. Following this, an n-type dopant is angle implanted with alower implant energy in a conventional manner without masking and drivenin to form an n-type side-wall region 532 within p− side-wall well 528.

In the present example, a p-type dopant is next implanted with a lowerimplant energy without masking through oxide layer 522 and driven in toform a thin p+ layer 534. After this, a p-type dopant is angle implantedwith a lower implant energy in a conventional manner without masking anddriven in to form a thin p+ side-wall layer 536.

The above dopant steps are illustrated without lithography, which canoptionally be utilized, for example, to block the formation of p-typeand n-type materials into the bottom surface of p-type bulk 510. P− well526, n-type region 530, and thin p+ layer 534 form upper photo detector222, while p− well 528, n-type region 532, and thin p+ side-wall layer536 form side-wall photo detector 224.

Following this, as shown in FIG. 5D, a diamond layer 540 is formed withconventional chemical vapor deposition on oxide layer 522 to line cavity524 and form a lined cavity 542. Diamond layer 540 is highly thermallyconductive (e.g., 5-10× more thermally conductive than copper). In thepresent example, oxide layer 522 and diamond layer 540 form heatspreader 218.

As shown in FIG. 5E, the method continues with the front-side processingby inverting SOI wafer 500, and then forming a mask 544 on the top sideof p-type active region 514 in a conventional manner. Following this, asshown in FIG. 5F, the exposed regions are etched until a number ofthrough-the-wafer openings 546 have been formed. Once thethrough-the-wafer openings 546 have been formed, mask 544 is removed ina conventional manner. (The through-the-wafer openings 546 expose bondpads after a subsequent dicing process.)

Next, as shown in FIG. 5G, a number of circuit elements 548 are formedin and on the top surface of p− active region 514 in a conventionalmanner. The circuit elements 548 include transistors, resistors,capacitors, diodes, and similar circuit devices. (For simplicity, onlyone NMOS transistor 548, which has an n-type source 548S and an n-typedrain 548D spaced apart by a p-type channel 548C, a gate oxide layer548G that touches the top surface of p− active region 514 over channel548C, and a gate 548R that touches oxide layer 548G over channel 548C,is illustrated.)

When electrically connected together, the circuit elements 548 form anelectronic circuit 548E that detects and amplifies the signals output byupper photo detector 222 and side wall photo detector 224. In addition,electronic circuit 548E can also compare the amplified signal from sidewall photo detector 224 to the amplified signal from upper photodetector 222, such as by using the amplified signal from side wall photodetector 224 and the amplified signal from upper photo detector 222 asthe inputs to a conventional differential pair circuit.

After the circuit elements 548 have been formed, as shown in FIG. 5H, amask 550 is formed on the top surface of p− active region 514 in aconventional manner. Following this, the exposed regions of p− activeregion 514 are etched away to form an opening that exposes p− well 526.A p-type material is then implanted through the opening to form a p+contact region 552 in p− well 526. Mask 550 is then removed in aconventional manner.

After mask 550 has been removed, as shown in FIG. 5I, a mask 554 isformed on the top surface of p− active region 514. Following this, theexposed regions of p− active region 514 are etched away to form anopening that exposes n-type region 530. An n-type material is thenimplanted through the opening to form an n+ contact region 556 in n-typeregion 530. Mask 554 is then removed.

After mask 554 has been removed, as shown in FIG. 5J, a mask 560 isformed on the top surface of p− active region 514. Following this, theexposed regions of p− active region 514 and insulator 512 are etchedaway to form an opening that exposes p− side-wall well 528 and anopening that exposes n-type side-wall region 532. Mask 560 is thenremoved.

After mask 560 has been removed, as shown in FIG. 5K, a mask 562 isformed on the top surface of p− active region 514 with an opening thatexposes p− side-wall well 528. A p-type material is then implantedthrough the opening to form a p+ contact region 564 in p− side-wall well528. Mask 562 is then removed.

After mask 562 has been removed, as shown in FIG. 5L, a mask 566 isformed on the top surface of p− active region 514 with an opening thatexposes n-type side-wall region 532. An n-type material is thenimplanted through the opening to form an n+ contact region 568 in n-typeside-wall region 532. Mask 566 is then removed to expose p+ contactregion 552, n+ contact region 556, p+ contact region 564, and n+ contactregion 568. Although the present example illustrates p+ contact region552, n+ contact region 556, p+ contact region 564, and n+ contact region568 each being exposed by a single opening, multiple openings can beused to expose each of these highly doped regions.

Following the removal of mask 566, as shown in FIG. 5M, an oxide layer570 is formed on the top surface of p− active region 514 to line theopenings that expose p+ contact region 552, n+ contact region 556,p+contact region 564, and n+ contact region 568. Following this, a mask572 is formed and patterned on oxide layer 570 to expose the regions ofoxide layer 570 that lie over p+ contact region 552, n+ contact region556, p+contact region 564, and n+ contact region 568. Mask 572 alsoexposes the regions of oxide layer 570 that lie over the circuitelements 548.

As shown in FIG. 5N, after mask 572 has been formed, the exposed regionsof oxide layer 570 are then etched to expose p+ contact region 552, n+contact region 556, p+ contact region 564, and n+ contact region 568. Inaddition, the circuit elements 548 are also exposed. Mask 572 is thenremoved.

After this, as shown in FIG. 5O, a photoresist layer 574 is depositedand patterned to cover the circuit elements 548 and thethrough-the-wafer openings 546, followed by the conventional depositionof a metal contact layer 576. Metal contact layer 576, which can includetungsten, lines the openings and makes electrical connections to p+contact region 552, n+ contact region 556, p+ contact region 564, and n+contact region 568. After metal contact layer 576 has been formed, amask 578 is formed on metal contact layer 576.

As shown in FIG. 5P, after mask 578 has been formed, the exposed regionsof metal contact layer 576 are etched to form a first metal contactstructure 580-1 that makes an electrical connection with p+ contactregion 552, and a second metal contact structure 580-2 that makes anelectrical connection with n+ contact region 556. The etch also forms athird metal contact structure 580-3 that makes an electrical connectionwith p+ contact region 564, and a fourth metal contact structure 580-4that makes an electrical connection with n+ contact region 568.Following the etch, mask 578 is conventionally removed, followed by theconventional removal of photoresist layer 574.

Oxide layer 570 and first metal contact structure 580-1 form first photodetector contact structure 226-1, while oxide layer 570 and second metalcontact structure 580-2 form second photo detector contact structure226-2, oxide layer 570 and third metal contact structure 580-3 formthird photo detector contact structure 226-3, and oxide layer 570 andfourth metal contact structure 580-4 form fourth photo detector contactstructure 226-4.

As shown in FIG. 5Q, following the removal of photoresist layer 574, aninterconnect structure 582 is conventionally formed on the top surfaceof p− active region 514 to electrically connect first metal contactstructure 580-1, second metal contact structure 580-2, third metalcontact structure 580-3, and fourth metal contact structure 580-4 toelectronic circuit 548E, and form a first fabricated wafer 583.

Interconnect structure 582 can be conventionally formed by depositing anoxide layer 582X, followed by a mask and etch step to form openings thatexpose first metal contact structure 580-1, second metal contactstructure 580-2, third metal contact structure 580-3, fourth metalcontact structure 580-4, and the conductive contacts of electroniccircuit 548E. Following this, a metal layer is deposited, and thenplanarized to remove the metal layer from the top surface of oxide layer582X and form contacts 582C in the openings.

After the metal contacts 582C have been formed, a metal layer isdeposited and then masked and etched to form a number of metal-1 traces582M. Following this, a passivation layer 582P is formed to touch oxidelayer 582X and the metal-1 traces 582M. Regions of passivation layer582P, which can be implemented with a layer of oxide and an overlyinglayer of nitride, are then removed to expose the bond pad regions of themetal-1 traces 582M.

As shown in FIG. 5R, optional mirror 234 can be formed by depositing alayer of metal 584, such as aluminum, on the top surface of interconnectstructure 582, followed by the formation of a mask 586. Once mask 586has been formed, the exposed regions of metal layer 584 are etched andremoved. Mask 586 is then removed to form mirror 234.

As shown in FIG. 5S, the upper RF coils 312 of atomic magnetometer 300can similarly be formed, by depositing a layer of metal 588, such asaluminum, on the top surface of interconnect structure 582, followed bythe formation of a mask 590. (The dashed portions of mask 590 indicatethat portions of the mask lie above and below the plane of the page.)

Once mask 590 has been formed, as shown in FIG. 5T, the exposed regionsof metal layer 588 are etched and removed. Mask 590 is then removed toform the four upper RF coils 312. Following this, a passivation layer592 is formed to touch passivation layer 582P and the four upper RFcoils 312. Regions of passivation layer 592, which can be implementedwith a layer of oxide and an overlying layer of nitride, are thenremoved to expose the bond pad regions of the upper RF coils 312.

Alternately, the upper RF coils 312 can be electrically connected to themetal-1 traces 582M by forming openings in passivation layer 582P toexpose regions of the metal-1 traces 582M before metal layer 588 isdeposited. In addition, the upper RF coils 312 can alternately be formedby way of a conventional electroplating process, which includesdeposition of a seed layer, followed by the formation of a plating moldwith four coil openings and electroplating.

As shown in FIG. 5U, upper ferrite block 410 of atomic magnetometer 400can be attached by applying an adhesive layer 594 to the top surface ofinterconnect structure 582 with a conventional pick-and-place machine,and then using a conventional pick-and-place machine to place ferriteblock 410 onto adhesive layer 594. Adhesive layer 594 can be implementedwith a conventional glue or die attach adhesive.

FIGS. 6A-6I show a series of cross-sectional views that illustrate anexample of a method of forming a second wafer in accordance with thepresent invention. As shown in FIG. 6A, the method utilizes aconventionally formed borosilicate glass wafer 600 that has a topsurface 600T and a bottom surface 600B. In the present example, wafer600 is approximately 700 μm thick, and polished on both sides. Themethod includes both front-side and back-side processing which can beperformed in either order.

As further shown in FIG. 6A, the method begins with the front-sideprocessing by forming a mask 610 on the top side of wafer 600 in aconventional manner. Following this, as shown in FIG. 6B, the exposedregions of wafer 600 are etched for a predetermined period of time toform a channel 612 that has a resistive section 612R with a narrowchannel width and bond pad sections 612B that each has a wide channelwidth. Once channel 612 has been formed, mask 610 is removed in aconventional manner.

Following this, as shown in FIG. 6C, a diffusion barrier layer 614 isdeposited onto the top surface of wafer 600 to line channel 612,followed by the deposition of an undoped polysilicon layer 616.Diffusion barrier layer 614 can be implemented with, for example, TaN orTa. Polysilicon layer 616 is deposited to a depth that fills the narrowchannel width of resistive section 612R, while only lining the widerbond pad sections 612B.

After polysilicon layer 616 has been formed, a metal layer 620 is formedon the top surface of polysilicon layer 616 to fill the remainingportions of the wider bond pad sections 612BP. Metal layer 620 can beformed by depositing a layer of metal or, alternately, by depositing acopper seed layer and then electroplating copper to form metal layer620.

As shown in FIG. 6D, once metal layer 620 has been formed, the topsurface of wafer 600 is planarized until the top surface of wafer 600 isexposed to form a heating element 622 that has a narrow resistivesection 622R of polysilicon and a wider bond pad section 622B of metalsurrounded by polysilicon. Following this, a diamond layer 624 ischemically vapor deposited onto the top surface 600T of wafer 600 andheating element 622 to form a portion of heat spreader 218.

As shown in FIG. 6E, the method continues with the back-side processingby inverting wafer 600, and forming a mask 626 on the bottom side 600Bof wafer 600 in a conventional manner. After mask 626 has been formed,as shown in FIG. 6F, the exposed region of wafer 600 is etched for apredetermined period of time to form a cavity 628. Mask 626 is thenremoved in a conventional manner.

Following this, as shown in FIG. 6G, a mask 630 is formed on the bottomsurface 600B of wafer 600 and cavity 628. After mask 630 has beenformed, as shown in FIG. 6H, the exposed region of wafer 600 is etchedfor a predetermined period of time to form a number of glue channels 632in the bottom surface of cavity 628. Mask 630 is then removed in aconventional manner.

As shown in FIG. 6I, after the front-side and back-side processing iscomplete, a glue drop 634 is inserted into each glue channel 632 using asyringe with a conventional pick-and-place machine, and an opticspackage 260 is placed into cavity 628 with a conventional pick-and-placemachine and attached to cavity 628 by way of glue drop 634 to form asecond fabricated wafer 636.

In addition, lower RF coil 310 of atomic magnetometer 300 can be formedat the same time that heating element 622 is formed, except that all ofthe channel that is etched in wafer 600 for lower RF coil 310 has thewider width so that lower RF coil 310 is implemented with metalsurrounded by polysilicon.

FIG. 7 shows a cross-sectional view that illustrates an example of theformation of a stacked wafer 700 in accordance with the presentinvention. As shown in FIG. 7, stacked wafer 700 is formed by firstplacing a substance 710, such as cesium azide, which can be decomposedby ultraviolet (UV) light into alkali and barrier atoms, into cavity 542of first fabricated wafer 583. Substance 710 can be applied with a watermixture to increase the accuracy of the amount of substance 710 that isdeposited. If substance 710 is applied with a water mixture, then wafer583 is dried to remove the water.

Next, second fabricated wafer 636 is aligned with first fabricated wafer583, and the two wafers are anodically bonded together in a conventionalmanner to close cavity 542, form a hermetically sealed vapor cell 712and stacked wafer 700. Following this, stacked wafer 700 is exposed toUV light which decomposes substance 710 into a gas having alkali andbarrier atoms.

FIG. 8 shows a cross-sectional view that illustrates an example of theformation of a stacked die 800 in accordance with the present invention.As shown in FIG. 8, stacked die 800 is formed by dicing stacked wafer700 so that the wafer saw passes through the through-the-wafer openings546. As a result, as further shown in FIG. 8, the dicing exposes thebond pads sections 622B, thereby making bond pad sections 622B availablefor subsequent wire bonding.

FIGS. 9A-9Q show a series of cross-sectional views that illustrate anexample of a method of forming a third wafer in accordance with thepresent invention. As shown in FIG. 9A, the method utilizes aconventionally formed p− single-crystal silicon wafer 900 that has a topsurface 900T and a bottom surface 900B. In the present example, wafer900 is polished on only one side.

As further shown in FIG. 9A, the method begins by forming a mask 910 onthe top side 900T of wafer 900 in a conventional manner. Following this,as shown in FIG. 9B, the exposed regions of wafer 900 are etched for apredetermined period of time to form a channel 912. Once channel 912 hasbeen formed, mask 910 is removed in a conventional manner.

Following this, as shown in FIG. 9C, an oxide layer 914 is formed on thetop surface 900T of wafer 900 to line channel 912, followed by thedeposition of an undoped polysilicon layer 916. Polysilicon layer 916 isdeposited to a depth that fills channel 912. As shown in FIG. 9D, oncepolysilicon layer 916 has been formed, the top surface 900T of wafer 900is planarized until the top surface of wafer 900 is exposed to form aheating element 922. Heating element 922 forms heating element 280.

Next, as shown in FIG. 9E, a mask 924 is formed on the top surface 900Tof wafer 900 and on heating element 922. After mask 924 has been formed,the exposed regions of p-type wafer 900 are implanted with a p-typedopant, such as boron, and then driven in to form a p− well 926. P-well926 has a dopant concentration that is greater than the dopantconcentration of p− wafer 900.

Once p− well 926 has been formed, an n-type material, such asphosphorous, is implanted with a lower implant energy and driven in toform an n-type region 930 in p− well 926. Following this, an n-typedopant is implanted with a lower implant energy and driven in to form athin n+ contact layer 932 and a temperature sensing diode 934. Mask 924is then removed in a conventional manner. Temperature sensing diode 934is temperature sensor 282.

As shown in FIG. 9F, a mask 936 is formed on the top surface 900T ofwafer 900, heating element 922, and diode 934 in a conventional manner.After mask 936 has been formed, the exposed regions of p− wafer 900 areetched for a predetermined period of time to form a first cavity 940 anda second cavity 942. Mask 936 is then removed in a conventional manner.Rather than being formed at the same time, the first and second cavities940 and 942 can be formed in separate masking and etching steps if thefirst and second cavities 940 and 942 require different depths.

Following this, as shown in FIG. 9G, a diamond layer 944 is chemicallyvapor deposited in a conventional manner on heating element 922, diode934, and the exposed regions of the top surface 900T of wafer 900 toline cavities 940 and 942. After this, as shown in FIG. 9H, a number ofglue drops 946 are inserted into the first and second cavities 940 and942 using a syringe with a conventional pick-and-place machine.Following this, VCSEL 286 and integrated circuit 290 are placed intocavity 940 and cavity 942, respectively, with a conventionalpick-and-place machine, and attached to cavity 940 and cavity 942,respectively, by way of the glue drops 946.

In addition, lower ferrite block 413 of atomic magnetometer 400 can beincorporated in the same way that VCSEL 286 and integrated circuit 290are incorporated, i.e., etch a third cavity when the first and secondcavities 940 and 942 are formed, and insert lower ferrite block 413 intoa glued third cavity with a pick-and-place machine.

As shown in FIG. 9I, after VCSEL 286 and integrated circuit 290 havebeen placed into and attached to cavity 940 and cavity 942,respectively, an epoxy layer, such as SU-8, is formed to touch diamondlayer 944 and fill the remaining areas of the first and second cavities940 and 942.

The epoxy layer is then exposed with a pattern and cured to form asubstantially planar non-conductive structure 950. The pattern forms anumber of openings in non-conductive structure 950, including a heatingelement opening 952-1, a temperature sensor diode opening 952-2, a laseropening 952-3 for the light output by VCSEL 286, a number of firstoperating openings 952-4 that expose the external pads of VCSEL 286, anda number of second operating openings 952-5 that expose the externalpads of integrated circuit 290.

As shown in FIG. 9J, after non-conductive structure 950 has been formed,a mask 954 is formed on non-conductive structure 950 to expose heatingelement opening 952-1 and temperature sensor diode opening 952-2. Aftermask 954 has been formed, as shown in FIG. 9K, the exposed portions ofdiamond layer 944 are etched to expose regions of heating element 922and n+ contact layer 932. Following this, mask 954 is removed in aconventional manner.

Next, as shown in FIG. 9L, a seed layer 960 is deposited to touchnon-conductive structure 950 and the exposed regions of heating element922, n+ contact layer 932, the external pads of VCSEL 286, and theexternal pads of integrated circuit 290. For example, seed layer 960 canbe formed by depositing 300 Å of titanium, 3000 Å of copper, and 300 Åof titanium. Once seed layer 960 has been formed, a plating mold 962 isformed on the top surface of seed layer 960.

As shown in FIG. 9M, following the formation of plating mold 962, thetop titanium layer is stripped and copper is deposited by electroplatingto form a number of metal-1 traces 964. As shown in FIG. 9N, after theelectroplating, plating mold 962 and the underlying regions of seedlayer 960 are removed to expose the metal-1 traces 964.

As shown in FIG. 9O, after the metal-1 traces 964 have been formed, apassivation layer 966 is formed on non-conductive structure 950 and themetal-1 traces 964. Passivation layer 966 can be formed in a number ofways. For example, passivation layer 966 can be implemented with a layerof oxide and an overlying layer of nitride, followed by an etch toexpose the laser light opening of VCSEL 286 and the regions of themetal-1 traces 964 that function as bond pads.

Alternately, an epoxy layer, such as SU-8, can be deposited, exposedwith a pattern, and then cured to form a substantially planarnon-conductive structure. The pattern forms a number of openings in thenon-conductive structure, including an opening that exposes the laserlight opening of VCSEL 286 and the regions of the metal-1 traces 964that function as bond pads.

As shown in FIG. 9P, after passivation layer 966 has been formed, a mask970 is formed on passivation layer 966. After mask 970 has been formed,the exposed portions of passivation layer 966 are etched to form gluechannels 972 in the top surface of passivation layer 966. Followingthis, mask 970 is removed in a conventional manner. As shown in FIG. 9Q,the removal of mask 970 forms a third fabricated wafer 974.

One of the advantages of the present invention is that the presentinvention integrates heating element 922, heat spreader 944, and VCSEL286 (which requires heat to be tuned to the proper light frequency) intoa single die-sized region. Further, the placement of VCSEL 286 in acavity and the use of a highly-thermally conductive heat spreader(diamond layer 944) to line the cavity allows more heat to be deliveredto VCSEL 286. In addition, the present invention utilizesthermally-sensitive diodes 934 to monitor the temperature of VCSEL 286.

The cross-sectional view shown in FIG. 9Q illustrates only onemagnetometer section of a large number of magnetometer sections that areformed on wafer 900. After the fabrication of third fabricated wafer974, a drop of glue, such as glue drop 273, is inserted into the gluechannels 972 with a conventional pick-and-place machine.

Following this, a stacked die 800 is placed onto each magnetometersection of wafer 900 with a conventional pick-and-place machine, andattached to each magnetometer section by way of the glue drops 273.After a stacked die 800 has been attached to each magnetometer sectionof wafer 900, wafer 900 is diced in a conventional manner to form theindividual atomic magnetometers 200, 300, and 400.

Thus, a die-sized atomic magnetometer and a method of forming thedie-sized atomic magnetometer have been described. One of the advantagesof the present invention is that the present invention provides a methodof forming atomic magnetometers that allows atomic magnetometers to bemass produced in conventional integrated circuit fabrication facilities.As a result, the present invention significantly reduces the cost ofatomic magnetometers. Another advantage of the present invention is thesmall size of the atomic magnetometers. Thus, the present inventionallows atomic magnetometers to be incorporated into many moreapplications than were possible with conventional magnetometers.

It should be understood that the above descriptions are examples of thepresent invention, and that various alternatives of the inventiondescribed herein may be employed in practicing the invention. Thus, itis intended that the following claims define the scope of the inventionand that structures and methods within the scope of these claims andtheir equivalents be covered thereby.

1. A magnetometer comprising: a first die having a top surface, a bottomsurface, and a first die cavity that extends into the first die from thebottom surface of the first die, the first die cavity having a topsurface and a side wall surface, the top surface of the first die cavitylying below and spaced apart from the top surface of the first die; anda second die having a top surface, a bottom surface, and a second diecavity that extends into the second die from the bottom surface of thesecond die, the second die cavity having a top surface and a side wallsurface, the top surface of the second die cavity lying below and spacedapart from the top surface of the second die, the top surface of thesecond die touching the bottom surface of the first die to close thefirst die cavity and form a hermetically sealed cell.
 2. Themagnetometer of claim 1 and further comprising a gas contained withinthe hermetically sealed cell, the gas including alkali atoms and bufferatoms.
 3. The magnetometer of claim 1 wherein the first die includes: afirst semiconductor structure having a top surface, a bottom surface,and a first semiconductor cavity that extends into the firstsemiconductor structure from the bottom surface of the firstsemiconductor structure, the first semiconductor cavity having a topsurface and a side wall surface, the top surface of the firstsemiconductor cavity lying below and spaced apart from the top surfaceof the first semiconductor structure; and a heat spreader that touchesthe bottom surface of the first semiconductor structure and the topsurface and the side wall surface of the first semiconductor cavity toline the first semiconductor cavity, the heat spreader being thermallyconductive.
 4. The magnetometer of claim 3 wherein the firstsemiconductor structure includes: an upper photo detector that liesvertically between the top surface of the first semiconductor structureand the top surface of the first semiconductor cavity, the first photodetector having a first well of a first conductivity type and a firstregion of a second conductivity type that touches the first well; and aside wall photo detector that lies laterally adjacent to the side wallsurface of the first semiconductor cavity, the side wall photo detectorhaving a second well of the first conductivity type and a second regionof the second conductivity type that touches the second well.
 5. Themagnetometer of claim 4 wherein the first semiconductor structurefurther includes: a first conductive structure that extends from the topsurface of the first semiconductor structure down into the firstsemiconductor structure to make an electrical connection with the firstwell; a second conductive structure that extends from the top surface ofthe first semiconductor structure down into the first semiconductorstructure to make an electrical connection with the second well; a thirdconductive structure that extends from the top surface of the firstsemiconductor structure down into the first semiconductor structure tomake an electrical connection with the first region; a fourth conductivestructure that extends from the top surface of the first semiconductorstructure down into the first semiconductor structure to make anelectrical connection with the second region; and an electronic circuitthat touches the top surface of the first semiconductor structure. 6.The magnetometer of claim 5 wherein the first die further includes aninterconnect structure that touches the top surface of the firstsemiconductor structure, the interconnect structure electricallyconnecting the electronic circuit to the first, second, third, andfourth conductive structures and to a number of external electricalconnections.
 7. The magnetometer of claim 6 wherein the first diefurther includes a mirror that touches a top surface of the interconnectstructure.
 8. The magnetometer of claim 6 wherein the first die furtherincludes an RF coil structure that touches a top surface of theinterconnect structure, the RF coil structure having a number of spacedapart coils that lie in a common plane.
 9. The magnetometer of claim 1wherein the second die includes an optics package attached to the topsurface of the second die cavity, the optics package including acircular polarizer.
 10. The magnetometer of claim 9 wherein the seconddie includes a second semiconductor structure having a top surface, abottom surface, and a second semiconductor cavity that extends into thesecond semiconductor structure from the bottom surface of the secondsemiconductor structure, the second semiconductor structure furtherincluding a coil that touches the second semiconductor structure. 11.The magnetometer of claim 1 and further comprising: a third die having atop surface, a bottom surface, and a third die cavity that extends intothe third die from the top surface of the third die, the top surface ofthe third die being attached to the bottom surface of the second die;and a laser attached to a bottom surface of the third die cavity. 12.The magnetometer of claim 3 wherein the heat spreader include a diamondlayer.
 13. A method of forming a magnetometer comprising: forming afirst die having a top surface, a bottom surface, and a first die cavitythat extends into the first die from the bottom surface of the firstdie, the first die cavity having a top surface and a side wall surface;forming a second die having a top surface, a bottom surface, and asecond die cavity that extends into the second die from the bottomsurface of the second die; and attaching the first die to the second dieso that the top surface of the second die touches the bottom surface ofthe first die and closes the first die cavity to form a hermeticallysealed cell.
 14. The method of claim 13 and further comprising forming agas within the hermetically sealed cell.
 15. The method of claim 14wherein the gas includes alkali atoms and buffer atoms.
 16. Asemiconductor structure comprising: a top surface; a bottom surface; asemiconductor cavity that extends into the semiconductor structure fromthe bottom surface of the semiconductor structure, the semiconductorcavity having a top surface and a side wall surface, the top surface ofthe semiconductor cavity lying below and spaced apart from the topsurface of the semiconductor structure; an upper photo detector thatlies vertically between the top surface of the semiconductor structureand the top surface of the semiconductor cavity, the first photodetector having a first well of a first conductivity type and a firstregion of a second conductivity type that touches the first well; and aside wall photo detector that lies laterally adjacent to and touches theside wall surface of the semiconductor cavity, the side wall photodetector having a second well of the first conductivity type and asecond region of the second conductivity type that touches the secondwell.
 17. A method of forming a semiconductor structure having a topsurface and a bottom surface, the method comprising: forming asemiconductor cavity that extends into the semiconductor structure fromthe bottom surface of the semiconductor structure, the semiconductorcavity having a top surface and a side wall surface, the top surface ofthe semiconductor cavity lying below and spaced apart from the topsurface of the semiconductor structure; forming an upper photo detectorthat lies vertically between the top surface of the semiconductorstructure and the top surface of the semiconductor cavity, the firstphoto detector having a first well of a first conductivity type and afirst region of a second conductivity type that touches the first well;and forming a side wall photo detector that lies laterally adjacent toand touches the side wall surface of the semiconductor cavity, the sidewall photo detector having a second well of the first conductivity typeand a second region of the second conductivity type that touches thesecond well.
 18. A semiconductor structure comprising: a top surface; abottom surface; a semiconductor cavity that extends into thesemiconductor structure from the bottom surface of the semiconductorstructure, the semiconductor cavity having a top surface and a side wallsurface, the top surface of the semiconductor cavity lying below andspaced apart from the top surface of the semiconductor structure; and anoptics package attached to the top surface of the semiconductor cavity,the optics package including a circular polarizer.
 19. A method offorming a semiconductor structure having a top surface and a bottomsurface, the method comprising: forming a semiconductor cavity thatextends into the semiconductor structure from the bottom surface of thesemiconductor structure, the semiconductor cavity having a top surfaceand a side wall surface, the top surface of the semiconductor cavitylying below and spaced apart from the top surface of the semiconductorstructure; and attaching an optics package to the top surface of thesemiconductor cavity, the optics package including a circular polarizer.20. A semiconductor structure comprising: a top surface; a bottomsurface; a semiconductor cavity that extends into the semiconductorstructure from the top surface of the semiconductor structure; and alaser attached to a bottom surface of the semiconductor cavity.
 21. Amethod of forming a semiconductor structure having a top surface and abottom surface, the method comprising: forming a semiconductor cavitythat extends into the semiconductor structure from the top surface ofthe semiconductor structure; and attaching a laser to a bottom surfaceof the semiconductor cavity.
 22. A method of operating an atomicmagnetometer comprising: generating a current flow through a lower wireand each of a plurality of upper wires; and varying an amplitude of thecurrent flow through the plurality of upper wires to steer a magneticfield axis of a magnetic field that results from the current flowthrough the lower wire and the plurality of upper wires.
 23. Asemiconductor device comprising: a first photo detector that measuresnon-absorption photons and emission photons that pass out of a vaporcell to determine a first total number of measured photons, andgenerates a first signal that represents the first total number ofmeasured photons; a first amplifier that amplifies the first signal togenerate an amplified first signal; a second photo detector thatsimultaneously measures substantially only emission photons that passout of the vapor cell to determine a second total number of measuredphotons, and generates a second signal that represents the second totalnumber of measured photons; a second amplifier that amplifies the secondsignal to generate an amplified second signal; and a differential paircircuit having a first input connected to receive the first amplifiedsignal, and a second input connected to receive the second amplifiedsignal, the differential pair circuit subtracting the amplified secondsignal from the amplified first signal to generate an output signal. 24.A method of operating a semiconductor device comprising: measuringabsorption photons and emission photons that pass out of a vapor cell todetermine a first total number of measured photons, and generating afirst signal that represents the first total number of measured photons;amplifying the first signal to generate an amplified first signal;simultaneously measuring substantially only emission photons that passout of the vapor cell to determine a second total number of measuredphotons, and generating a second signal that represents the second totalnumber of measured photons; amplifying the second signal to generate anamplified second signal; and subtracting the amplified second signalfrom the amplified first signal to generate an absorption signal.